Surgical materials are important synthetic biomaterials that can be implanted in humans or animals and are used extensively in orthopedic surgery and related areas such as total joint arthroplasty. A number of dental applications also require the use of surgical materials such as cements.
The most common cement currently employed in cemented total joint arthroplasty is obtained by the polymerization of poly(methyl methacrylate) with methyl methacrylate monomer. In cemented total joint replacement, the surgical cement, also referred to as the bone cement, anchors the prosthesis to the contiguous bone. One concern associated with self-curing acrylic-based bone cements is the fracture of cements due to defects such a voids and agglomeration of fillers. (Topoleshi LDT. et al, Biomaterials 14(15): 1166-1172 (1993) Microstructual Pathway of Fracture in Poly(Methyl Methacrylate Bone Cement (Demian, et al. Regulatory Perspective on Characterization And Testing of Orthopedic Bone Cement, Biomaterials 19: 160-7-1618 (1998). Accordingly, a need exists for surgical cements which eliminate or minimize the above-referenced problems.
The invention is directed to composite surgical materials characterized by a very fine matrix ligament thickness. The surgical material is referred to herein as a xe2x80x9cnanocomposite materialxe2x80x9d as the material is characterized by a nanoscale structure produced by nanometer fillers. However, micron size fillers can be used in the materials and methods described herein. The average matrix ligament thickness of the composite surgical materials of the invention is less than about 1000 nanometers, preferably less than about 750 nanometers. More preferred are average matrix ligament thicknesses of less than about 500 nanometers. In an especially preferred embodiment, the surgical materials of the invention have an average matrix ligament thickness of less than about 250 nanometers. The reduced matrix ligament thickness of the surgical materials of the invention is promoted by both (a) fillers having a very fine average mass diameter and (b) dispersion methods which avoid or preclude filler aggregation or agglomeration.
The composite surgical materials of the invention include a polymer matrix (e.g., biodegradable or non-biodegradable polymer) and filler. In a preferred embodiment, the polymer matrix is obtained by the polymerization of poly(methyl methacrylate) with methyl methacrylate monomer. Other polymeric materials can also be employed.
The filler can be solid, liquid, gas or mixture thereof. The filler has an average mass diameter less than about 1000 nanometers, preferably less than about about 750 nanometers. More preferred are fillers having an average mass diameter of less than about 500 nanometers. In most preferred embodiments, fillers have an average mass diameter between about 1 nanometer and about 250 nanometers and in particular between about 1 nanometer and about 150 nanometers.
The invention also pertains to a polymeric powder for preparing a nanocomposite surgical material as described herein. A polymeric precursor (powder) is admixed with a filler to produce a uniformly dispersed polymeric powder. The powder can then be conveniently used in the preparation of a surgical material or cement. Preferably the polymeric powder includes polymethyl methacrylate.
The invention is also related to methods and apparatus for incorporating a filler in a polymer matrix and forming a composite surgical material having the reduced matrix ligament thickness discussed above. For an acrylic type surgical cement, the filler can be introduced into one of the cement precursors prior to mixing, during the mixing of the precursors or into a pre-prepared paste, dough or liquid where polymerization is induced. The filler is combined, dispersed or mixed into the cement in a way which minimizes the aggregation, agglomeration or coalescence of filler particles or filler domains such as filler droplets or bubbles.
One aspect of the invention is related to a method and apparatus wherein air-cement contact may be eliminated during storage, during transfer to the mixing chamber, during the mixing process, during the transfer from the mixing vessel to the syringe and to the nozzle, as well as during release at the in vivo site. In preferred embodiments of the invention, the methods and apparatus are used to incorporate the filler into the composite surgical materials discussed above.
The invention has many advantages. For example, the composite surgical material (e.g., surgical cements, plates and screws) of the invention have superior mechanical properties, in particular good toughness. The surgical materials prepared using the methods and apparatus of the invention are less likely to form the type of large voids which are thought to result in surgical material prone to fractures. The invention allows for the incorporation of small size, well dispersed fillers into the surgical material (e.g., surgical cements, bone cements, plates and screws). Fillers other than solid particles can be introduced into the composite surgical materials by practicing the invention. Since the invention can provide for the isolation or occlusion from ambient air of all steps involved in preparing and transferring the surgical material into a human or veterinarian patient, escape of harmful vapors or odors into the work environment is reduced or eliminated. If contact with air occurs at any of the steps, vacuum or a pressurizing gas can be reapplied.